The vast majority of Americans do not engage in enough physical activity (PA) and rates of participation are particularly low for women aged 45 years or older. The association between regular PA and reduced risk for breast cancer is well established with some estimates suggesting that physically active women may be as much as 80% less likely to develop breast cancer compared to physically inactive women. For this reason, there is great public health incentive to increase PA rates among middle-aged women. How best to motivate PA behavior is an area of study in need of a more concrete theoretical framework. Research conducted across several broad areas of psychology including, clinical, social, and health have consistently implicated self-monitoring as an effective intervention for increasing participation in health behaviors (i.e., PA). In terms of what aspects of the PA experience might be most useful to self-monitor, recent work suggests that among the middle-aged female demographic, increased awareness of the immediate benefits PA holds for everyday life (i.e., affect improvement) may be important. The PA-affect relationship may be an especially relevant focus as more favorable affect during and immediately following exercise has been linked to subsequently greater PA intentions and more frequent exercise behavior over time. A crucial limitation of past work studying the PA-affect relationship has been the inherent inability to study the relationship using an experimental design. A specific aim of the proposed study is to address this gap in the literature. Although it is not possible to randomly assign an individual to have a pre-specified affective response to PA, it is possible to randomly assign an individual to attend to a pre-specified aspect of the PA experience through the use of self-monitoring. Specifically, this study will assign women aged 40-60 to self-monitor (by completing daily PA-journals) aspects of the PA experience that are psychological in nature (i.e., affect), physiological in nature (e.g., calories burned etc.), or neutral (i.e., self-reportonly the activity that was performed). The self-monitoring intervention will take part during first mont of study participation only - thereafter, participants will complete follow-up assessments at 3 and 6-months post-baseline that will assess levels of PA participation and other psychosocial variables. In addition to testing the effectiveness of the self- monitoring intervention, an exploratory aim of this project is to examine the mechanisms by which the self- monitoring interventions influenced behavior change and maintenance through a meditational analysis utilizing constructs from a novel theoretical model of PA, the Theory of Physical Activity Maintenance (PAM). The long-term objectives of this project are (1) to better understand the factors that motivate and maintain PA behavior among middle-aged women; and (2) to inform the design of future PA-based interventions and clinical trials focused on promoting women's health generally, and breast cancer prevention efforts specifically.